The present invention relates to the field of wireless communications. More specifically, the present invention relates to efficient recovery of buffered data following handover in a wireless system that distributes data from an intermediate point. Third generation (3G) systems having a remote network controller (RNC) which is coupled to one or more Node Bs, which are in turn wirelessly coupled to a plurality of User Equipment (UEs), employing adaptive modulation and coding (AMC) and hybrid automatic repeat request (H-ARQ) techniques are just one example of this kind of system.
A 3G Universal Terrestrial Radio Access Network (UTRAN) comprises several RNCs, each of which can be coupled to several Node Bs. A node B is an entity comprising one or more base stations, each managing traffic for one or more cells.
The 3G FDD and TDD systems typically use the RNC to buffer and schedule data transmissions to the UE. However, for the high speed channels of 3G cellular systems, data is buffered and scheduled for transmission by a Node B. One of these high speed channels, for example, is the High Speed Downlink Shared Channel (HS-DSCH). Since data is distributed by the Node B, it is necessary to buffer data for transmission in Node B. A result of this architecture is the RNC does not have an up-to-date status of the Node-B controlled transmissions of Packet Data Units (PDU). Therefore, upon handover between cells, it is necessary to coordinate the scheduling of data transmissions with the cell change. Otherwise following the cell change it is necessary to resynchronize data transmission to avoid loss or duplication of transmitted data. In 3G networks handover between cells is controlled by the RNC. Since the RNC that controls the cell the UE is in may change there is the possibility data may be lost or duplicated as a result of the handover. This problem is aggravated due to the architecture whereby there are several Node Bs associated with each RNC. There is a much higher likelihood that a mobile UE will require a Node B change than a change of RNC as a result of UE cell handovers.
The HS-DSCH utilizes AMC to enable high speed transmission of data and H-ARQ to increase the possibility of successful delivery of data. A serving HS-DSCH cell change is when the-UE has to change the cell associated with the UTRAN access point that is performing transmission and reception of the serving HS-DSCH radio link. The serving HS-DSCH cell change is invoked when improved physical channel conditions and/or improved physical capacity is realized in an alternate cell. Unlike other channels in 3G networks that terminate at RNC within the UTRAN, the HS-DSCH terminates at Node B.
There are two types of HS-DSCH cell changes. An Intra-Node B serving HS-DSCH cell change is when the UE changes between two cells that are associated with the same Node B. An Inter-Node B serving HS-DSCH cell change is when the UE changes between two cells that are associated with different Node Bs. In an Inter-Node B cell change, the Node B before the serving HS-DSCH cell change is called the source Node B, and the Node B after the serving HS-DSCH cell change is called the target Node B.
There are peer Radio Link Control (RLC) entities in both the RNC and the UE providing an automatic repeat request (ARQ) function for transmission of data. The sending RLC entity signals a sequence number (SN) in the PDU header, which is used by the receiving RLC entity to ensure that no PDUs are missed in the transmission. If there are PDUs missed during the transmission, realized by out-of-sequence delivery of PDUs, the receiving RLC entity sends a status report PDU to inform the sending RLC entity that certain PDUs are missing. The status report PDU is used to describe the status of the data transmission. It identifies the SNs of the PDUs that are missed or received. If a PDU is missed, the sending RLC entity will retransmit a duplicate of the missed PDU to the receiving RLC.
It is also possible for the sending RLC entity to poll for a status report PDU from the receiving RLC entity, or to generate status reports periodically. The polling function provides a mechanism for the sending RLC entity to request the status of PDU transmissions.
A H-ARQ function in the Node B also provides for retransmission of failed transmissions. Although the H-ARQ operation removes some failed transmissions and increases the probability of successful delivery of data, it is the RLC protocol layer that ultimately ensures successful delivery.
Due to dynamic changes in propagation conditions, the HS-DSCH cell change must be performed rapidly to maintain quality of service. During the serving HS-DSCH cell change, it is possible that the UE stops transmission and reception in the source cell before all PDUs currently stored in the source Node B are successfully transmitted. Since the source Node B performs scheduling and buffering of the data, and since the data rates are very high, (for example 10 Mb/sec or higher), when the UE performs a serving HS-DSCH cell change, (especially for an Inter-Node B handover), there is a possibility that considerable amounts of data buffered in the source Node B will be lost. One reason for this data loss is that no mechanism exists within the UTRAN architecture for data buffered at the source Node B to be transferred to the target Node B. Upon serving a HS-DSCH cell change, the RNC has no information on how much, if any, data is lost since the RNC is not informed of Node B data transmission scheduling and if the transmission have been successfully acknowledged by the UE. Therefore when data is buffered in the source Node B upon serving HS-DSCH cell change to maintain transmission without loss of data, the RNC RLC must recover this data.
There are currently two preferred ways that prior art systems handle the recovery of data buffered at the source Node B. Following the HS-DSCH cell change: 1) the RNC can explicitly request a status PDU from the UE; or 2) the RNC can just start transmitting where it stopped in the source cell and out-of-sequence delivery realized by the UE will generate the status PDU.
In the first case, where the RNC explicitly requests a status PDU by polling the UE, the RNC must first wait until the physical channel is established in the new cell. The status PDU request is then sent and is received and processed by the UE. The UE generates the status PDU and sends it back to the RNC, which processes the status PDU and determines which PDUs are in need of retransmission.
In the second case, where the RNC just starts transmitting PDUs from where it stopped in the source cell, the UE recognizes the out-of-sequence delivery of data and generates a status PDU back to the RNC. The RNC processes the status PDU and learns which PDUs are in need of retransmission.
In either of these two cases, if data buffered in the source Node B needs to be recovered, then a status PDU will be processed, but proper reception of retransmitted data by the UE will be considerably delayed. This is due to delayed generation of the status PDU by the UE and reception of the status PDU in the RNC.
If transmission is being performed in RLC acknowledged mode, data is not passed to higher layers until in-sequence delivery of data can be performed. Accordingly, the UE will be required to buffer the out-of-sequence data until the missing PDUs can be retransmitted. This not only results in a delay of the transmission, but requires the UE to have a memory which is sufficiently large enough to buffer data until the data stored in the source Node B can be successfully delivered. Otherwise, the effective data transmission rate is reduced, thereby effecting quality of service. Since memory is very expensive, this is an undesirable design constraint.
Accordingly, the prior art methods of recovering data that is buffered in a source Node B prior to transfer to a target Node B have very undesirable consequences. It is desirable to have a system and method where data buffered in the source Node B can be more efficiently recovered with less delay to properly maintain user quality of service requirements.
The present invention is a method and system which implement a series of actions in order to reduce latency, and potentially prevent loss of PDU transmissions during the serving HS-DSCH cell change procedure. A new criterion is introduced for UE generation of the status report of RLC PDUs. The UE autonomously generates status reports of the PDUs as soon as possible following notification of the HS-DSCH cell change indicated by a Radio Resource Control (RRC) handover procedure to more efficiently recover source Node B buffered data. PDU status may be signaled for each acknowledged mode (AM) RLC instance associated with the HS-DSCH transport channel.